1. Field of the Invention
The present invention relates to a welder monitor for monitoring the welding quality during resistance welding and, in particular, during spot welding.
2. Description of Related Art
In general, conventional welder monitors cannot judge the time at which electrode chips should be replaced or repaired at a welding site, and welder maintenance is mechanically carried out when the number of welding times reaches a predetermined one. For this reason, the electrode chips being still usable are often replaced with new ones at an earlier stage than necessary, or unnecessary repairs are made.
To overcome these problems, the inventors of this application have proposed a welder monitor as shown in FIG. 7.
In FIG. 7, reference numeral 20 denotes a detecting section for detecting a welding current and a welding voltage. The detecting section 20 detects welding current signals using a toroidal coil 29 mounted on an electrode 27 and measures the welding current by integration of such welding current signals. The detecting section 20 also measures a welding voltage applied between two electrodes 27 and 28 using associated voltage detection lines 30 connected thereto. Reference numeral 21 denotes an operation section for estimating the diameter of a growing nugget based on a temperature distribution of materials 31 to be welded and that of each electrode 27 and 28, both of which are estimated for each unit time. Upon receipt of measurement results from the detecting section 20, the estimation of the temperature distribution is carried out by a numerically-computing simulator based on a heat conduction model. A numerically-computing simulator for numerically computing, based on a heat conduction model, a temperature distribution and a diameter within which electricity flows using measured welding current and welding voltage is disclosed, for example, on pages 12 to 52 of "Resistance Welding Phenomenon and its applications (I)" edited by a resistance welding research commission in the Welding Society.
Reference numeral 22 denotes a reference diameter setting section for setting the nugget diameter to be used as a reference value. Reference numeral 23 denotes a nugget diameter comparing section for comparing the estimated nugget diameter computed by the operation section 21 with the set nugget diameter set by the reference diameter setting section 22, to thereby compute the time at which the estimated nugget diameter exceeds the set nugget diameter (this time is hereinafter referred to as an estimated nugget formation time). Reference numeral 24 denotes a reference time setting section for setting the nugget formation time to be used as a reference time. Reference numeral 25 denotes a time comparing section for comparing the estimated nugget formation time computed by the nugget diameter comparing section 23 with the nugget formation time set by the reference time setting section 24. Reference numeral 26 denotes an output section for outputting a comparison result obtained by the time comparing section 25.
With the above-described construction, based on the welding current and welding voltage detected by the detecting section 20, the operation section 21 computes the estimated nugget diameter using the numerically-computing simulator. Then, the nugget diameter comparing section 23 compares the estimated nugget diameter with the set nugget diameter (reference diameter), thereby computing the estimated nugget formation time. The time comparing section 25 compares the estimated nugget formation time with the set nugget formation time (reference time), and a comparison result is outputted from the output section 26.
The welder monitor referred to above makes use of the fact that the nugget formation time becomes gradually longer with damage of the electrodes 27 and 28, and estimates this time using the heat conduction model. By comparing the estimated nugget formation time with the set reference one, the welder monitor informs an operator of the maintenance time at which the welding conditions should be reset or the electrode chips should be replaced or repaired.
FIG. 8 depicts a graph indicating a relationship between the weld time and the estimated nugget diameter. As shown therein, if an object to be welded having a thickness less than a predetermined one is erroneously fed to the welding site for some reason, the estimated nugget formation time is shifted to the side shorter in the weld time. In this case, even if the electrodes 27 and 28 are damaged, the welder monitor cannot inform the operator of the appropriate maintenance time or the necessity of resetting the welding conditions.
Furthermore, even if erroneous detection or abnormal welding occurs, the welder monitor is likely to erroneously judge that the maintenance time has come.